GtkWindow: keep a reference of the widget on focus change.

Focus out could lead to the widget being destroyed (for example,
in editable cell renderers when DnD starts), so keep the widget
alive while iterating through the devices.

gtk/gtkwindow.c

@@ -5253,6 +5253,8 @@ do_focus_change (GtkWidget *widget,
   GdkDeviceManager *device_manager;
   GList *devices, *d;
 
+  g_object_ref (widget);
+
   device_manager = gdk_display_get_device_manager (gtk_widget_get_display (widget));
   devices = gdk_device_manager_list_devices (device_manager, GDK_DEVICE_TYPE_MASTER);
   devices = g_list_concat (devices, gdk_device_manager_list_devices (device_manager, GDK_DEVICE_TYPE_SLAVE));
@@ -5289,6 +5291,7 @@ do_focus_change (GtkWidget *widget,
     }
 
   g_list_free (devices);
+  g_object_unref (widget);
 }
 
 static gint